This invention relates to alumina refractories and methods for treating alumina refractories. More particularly, the invention relates to methods of improving the strength of alumina refractories and refractories having improved strength.
Zircon (ZrSiO4) and zirconia (ZrO2) are recognized as highly refractory materials. Accordingly, shaped bodies of such materials, such as brick, are frequently employed in conjunction with high temperature processes. Zircon is more frequently used because of its stability, ease of fabrication and lower cost.
Alumina is another known refractory material, and alumina is used in a variety of refractory applications, for example, in furnaces. Furnaces that are used in the manufacture of fused silica optical members, however, typically utilize zircon refractory materials. FIG. 1 shows a furnace 100 for producing fused silica glass. The furnace includes a crown 12 and a plurality of burners 14 projecting from the crown. As noted above, silica particles are generated in a flame when a silicon containing raw material together with a natural gas are passed through the plurality of burners 14 into the furnace chamber 26. These particles are deposited on a hot collection surface of a rotating body where they consolidate to the solid, glass state. The rotating body is in the form of a refractory cup or containment vessel 15 having lateral walls 17 and a collection surface 21 which surround the boule 19 and provide insulation to the glass as it builds up. The refractory insulation ensures that the cup interior and the crown are kept at high temperatures.
A standard fused silica furnace further includes a ring wall 50 which supports the crown 12. The furnace further includes a rotatable base 18 mounted on an oscillation table 20. The base is rotatable about an axis 3. The crown 12, the ring wall 3, the base 18 and the lateral walls are all made from suitable refractory materials, typically zircon refractory materials. Zircon refractories used in fused silica furnace must contain low levels of metallic impurities, and one way of reducing the levels of impurities is through a carbochlorination process, which is described in U.S. Pat. No. 6,174,509.
Although the process for treating zircon refractories described in U.S. Pat. No. 6,174,509 produces refractories that have a substantially lower levels of metallic impurities than untreated zircon refractories, there continues to be a need for refractory materials that introduce even lower levels of impurities to materials produced in the furnace. In addition, any improvement in the mechanical or thermal properties of a refractory material is of interest in not only fused silica furnaces, but for other uses as well. A refractory material having improved strength would find use in a wide variety of applications.
One embodiment of the invention relates to methods for improving the strength of alumina refractory materials. According to one embodiment of the invention, a method for improving the strength of alumina refractory materials is provided which includes the step of exposing the alumina refractory material to a halogen gas. In another embodiment, the step of exposing the alumina refractory to a halogen gas is performed at a temperature above about 1000xc2x0 C. In another embodiment, the temperature during the step of exposing the alumina refractory is performed at a temperature between about 1000xc2x0 C. and 1400xc2x0 C. In still another embodiment, the step of exposing the alumina refractory is performed in the presence of a reducing agent, such as carbon black or graphite.
According to still another embodiment, the halogen gas is selected from the group consisting of fluorine, iodine, and chlorine. In another embodiment, the halogen gas includes a mixture of chlorine and a carrier gas, such as, for example helium, nitrogen, hydrogen or argon. Another embodiment involves exposing the refractory material to a halogen gas for at least one hour and less than 10 hours. According to one embodiment of the invention, the four point bend strength of the alumina refractory material is improved by at least 50% by the exposure to the halogen gas.
The alumina refractories produced according the methods of the present invention may be used in a wide variety of applications. For example, according to one embodiment, a furnace for the production of fused silica optical members is provided that includes an alumina refractory material processed according to methods of the present invention.
Additional features and advantages of the invention will be set forth in the following detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.